Geochemistry of granitoids of Bilgi area,northern part of eastern Dharwar craton,southern India — Example of transitional TTGs derived from depleted source |
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Authors: | Sukanta Dey A K Rai Anjan Chaki |
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Institution: | (1) Present address: Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad, 500 016, India;(2) Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, Nagarabhavi, Bangalore, 560 072, India;(3) Department of Applied Geology, Indian School of Mines University, Dhanbad, 826 004, India |
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Abstract: | Mildly deformed granitoids exposed around Bilgi in the northernmost part of the eastern Dharwar craton are divided into two
groups viz. granodiorites and monzogranites. The granodiorites contain microgranular enclaves and amphibolite xenoliths, and
show low-Al TTG affinity with high SiO2 (71–74 %), Na2O, Y and Sr/Y, moderate to moderately high Mg#, Cr and Ni, low to moderate LILE, and low Nb and Ta. However, compared to similar
TTGs from different cratons the Bilgi granodiorites have distinctly higher K2O, K2O/Na2O, Rb and lower REE and Th. The amphibolite xenoliths are characterized by variable enrichment of K2O, Rb, Ba and Th and depletion of Ti, Zr and P compared to MORB. The microgranular enclaves are quartz diorite to granodiorite
in composition with high Mg, Ni and Cr, and compared to MORB, are enriched in LILE and depleted in Ti and Y. The monzogranites,
compared to the granodiorites, display higher SiO2, K2O and Rb with lower Mg#, although still maintaining the high Na2O, Ni and Cr and low REE character.
The Bilgi granodiorites are explained as transitional TTGs late synkinematic with respect to regional deformation. Geochemical
signatures and regional geological set up suggest that they are probably derived from partial melting of a highly depleted
slab material (metabasalt) followed by variable contamination or assimilation of intermediate crustal rocks in a subduction
zone set up. Late stage fluid activity on the granodioritic magma is probably responsible for the generation of monzogranites.
The amphibolite xenoliths predate the granodiorites and possibly represent fragments of a schist belt carried away by the
granitic magma. They are probably island arc basalt derived from mantle source that has been metasomatized by slab-derived
fluids. The microgranular enclaves are coeval with the Bilgi granodiorites and also likely to be island arc magmas derived
from mantle variably enriched in slab-derived and within-plate components. |
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Keywords: | Transitional TTG Petrography Geochemistry Petrogenesis Geodynamic setting Dharwar craton |
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